1. Field of the Invention
The present invention generally relates to a rechargeable lithium secondary battery suited for use in a mobile DC power unit, a back-up power unit. More particularly, the present invention relates to an improvement in lithium secondary battery comprising a non-aqueous electrolyte containing lithium ions, a positive electrode made of vanadium pentoxide and a negative electrode made of a lithium-aluminum alloy capable of reversibly absorbing and discharging lithium when the battery is charged and discharged.
2. Description of the Prior Art
The secondary battery utilizing lithium as a negative electrode is recently being largely developed in a number of institutes because of its reliability and possession of high density energies. By way of example, the applicability of vanadium oxides, chromium oxides, manganese oxides, molybdenum oxides, chalcogen compounds such as titanium disulfide and molybdenum disulfide, and electroconductive polymers such as polyacetylene and polyaniline as material for a positive electrode of the lithium secondary battery has been examined.
On the other hand, where lithium itself is used as material for the negative electrode, repetition of charging and discharge of the lithium secondary battery results in the generation of dendrite which in turn results in considerable reduction in cycle time between the charging and discharge. Accordingly, attempts have hitherto been made of the use of an aluminum alloy capable of absorbing lithium or a fusible alloy such as lead, bismuth, cadmium, indium and zinc.
The use of vanadium pentoxide or equivalent oxide as material for the positive electrode as is the case with the present invention is disclosed in, for example, U.S. Pat. Nos.3,929,504 and 4,228,226. The use of the lithium-aluminum alloy as material for the negative electrode is disclosed in, for example, U.S. Pat. No.4,002,492.
However, the lithium secondary battery wherein a particular combination of the above listed materials for the positive and negative electrodes is employed has not yet been used in practice.
When the secondary battery comprising the positive electrode made of vanadium pentoxide, the negative electrode made of pure lithium and the electrolyte made of a nonaqueous solvent dissolved with lithium perchloric acid is discharged, the discharge starts at about 3.5 volts with the voltage progressively decreasing down to about 3.0 volts while exhibiting a characteristic curve having a moderate flatness. During the period in which the voltage of the battery decreases from about 3.5 volts down to about 3.0 volts, the voltage corresponding to about one electron equivalent (the electric capacity of one mole of monovalent lithium which reacts with V.sub.2 O.sub.5) is discharged. Thereafter, the voltage quickly decreases and, when the voltage attains about 2.5 volts, a second-stage discharge starts. During this second-stage discharge taking place, the voltage drop takes place slowly until it attains about 2.0 volts with the electric capacity smaller than about one electron equivalent being further discharged.
Upon the decrease of the voltage subsequent to the second-stage discharge, the voltage further decreases accompanied by a third-stage reaction. During the third-stage reaction taking place, the voltage slowly decreases down to about 1.0 volt and then rapidly decreases down to zero volt. When the voltage of the battery is at a potential lower than about 1.0 volt, an organic solvent such as propylene carbonate is decomposed accompanied by generation of gases.
At about zero volt, crystals of vanadium pentoxide are pulverized to such an extent that the battery can no longer be rechargeable.
As discussed above, where vanadium pentoxide is used as material for the positive electrode, the continued discharge at a voltage lower than about 1.0 volt brings about the above discussed problem and, therefore, any countermeasures have hitherto been required to avoid the above discussed problem.